1. Field of the Invention
The present invention relates in general to a braking system for a motor vehicle, and more particularly to improvements in technology for optimizing an operation of a pump used in the braking system.
2. Discussion of the Related Art
There is known a braking system wherein the pressure in a brake cylinder is electrically controllable. One kind of such a braking system includes (a) an operator-controlled brake operating member, (b) a master cylinder for generating a fluid pressure according to an amount of operation of the brake operating member, (c) a wheel brake cylinder for operating a wheel brake to brake a wheel of a motor vehicle according to a fluid pressure applied to the wheel brake cylinder, (d) a reservoir for storing a working fluid, (e) a pump for pressurizing the fluid sucked up from the reservoir, and delivering the pressurized fluid toward at least one of the master cylinder and the wheel brake cylinder, (f) a DC power source, (g) a pump actuator driven by the DC power source to operate the pump, and (h) a solenoid-operated pressure control device having a plurality of operating states which are selectively established to electrically control the fluid pressure in the wheel brake cylinder and which include a pressure-increase state for substantial fluid communication of the wheel brake cylinder with at least one of the master cylinder and the pump, and a pressure-decrease state for substantial fluid communication of the wheel brake cylinder with the reservoir.
In one type of the braking system as described above, the solenoid-operated pressure control device is adapted to increase the fluid pressure in the wheel brake cylinder by permitting a flow of the working fluid from the master cylinder as a pressure source toward the wheel brake cylinder. In another type of the braking system, the solenoid-operated pressure control device is adapted to increase the fluid pressure in the wheel brake cylinder by inhibiting a flow of the working fluid from the master cylinder toward the wheel brake cylinder while permitting a flow of the working fluid from the pump as a pressure source toward the wheel brake cylinder. In the former type, the pump is used to return the fluid to the master cylinder from the reservoir which receives the fluid discharged from the wheel brake cylinder when the fluid pressure in the wheel brake cylinder is reduced. In the latter type, the pump is used to return the fluid from the reservoir to the wheel brake cylinder to increase the fluid pressure in the wheel brake cylinder. The former and latter types of braking system will be referred to as "fluid recirculating type" and "pressure-increase-by-pump type", respectively
An example of a known braking system of the pressure-increase-by-pump type is disclosed in Japanese publication JP-A-63-31870, which includes a reciprocating type solenoid-operated pump actuator, and a pump actuator control device having (i) a power supply portion for intermittently supplying electric power from a DC power source to a solenoid of the solenoid-operated pump actuator such that a duty ratio of the solenoid is controlled in response to a duty ratio signal, and (ii) a signal generating portion for generating the duty ratio signal. The signal generating portion includes a signal determining means for determining the duty ratio signal depending upon a friction coefficient of a road surface (on which the vehicle is running) when the fluid pressure in the wheel brake cylinder is controlled by the solenoid-operated pressure control device. In the braking system disclosed in the above-identified publication, therefore, the amount of delivery of the pressurized fluid from the pump is changed depending upon the friction coefficient of the road surface.
The inventors of the present application has developed a braking system of the pressure-increase-by pump type wherein the solenoid-operated pressure control device has a plurality of operating states which are selectively established to control the fluid pressure in the wheel brake cylinder and which include a pressure-decrease state in which the wheel brake cylinder communicates with not only the reservoir but also the pump. This braking system will be described in detail in the PREFERRED EMBODIMENT OF THE INVENTION.
The braking systems of the recirculating type as well as the pressure-increase-by-pump type may be provided with a diagnostic device for diagnosing the pump actuator for any abnormalities such as electrical disconnection or discontinuity of the pump actuator and associated components, and a failure of the pump actuator to normally operate or function. The pump actuator may be a rotary actuator in the form of an electric motor. In this case, the diagnostic device is adapted to supply electric power from the DC power source to the motor to hold the motor on for a predetermined time, and diagnose the motor and the associated components for abnormalities.
In the conventional braking systems described above, however, the operation of the pump is not sufficiently optimized during operation of the solenoid-operated pressure control device or diagnostic device, as described below in detail.
The braking system of the pressure-increase-by-pump type disclosed in the Japanese publication identified above is designed to change the delivery amount of the pump depending upon the friction coefficient of a road surface on which the vehicle is running. Therefore, the rate of increase of the fluid pressure in the wheel brake cylinder can be optimized depending upon the friction coefficient, namely, depending upon the fluid pressure in the wheel brake cylinder. However, the rate of increase of the fluid pressure in the wheel brake cylinder is changed by other factors. For example, the operation of the pump and the rate of increase of the wheel brake cylinder pressure are influenced by a variation in the output state (e.g., voltage) of the DC power source. Thus, the braking system disclosed in the publication suffers from a problem that a variation in the output state of the DC power source prevents an optimum rate of increase of the fluid pressure in the wheel brake cylinder.
In the braking system developed by the present inventor wherein the solenoid-operated pressure control device has a plurality of selectively established operating states including the pressure-decrease position in which the wheel brake cylinder communicates with not only the reservoir but also the pump, the pressurized fluid delivered from the pump is undesirably supplied to the wheel brake cylinder even when the pressure control device is placed in the pressure-decrease state. Therefore, if the duty ratio of the pump actuator driven by the DC power source when the pressure control device is in the pressure decrease state is the same as when the pressure control device is in the other operating states, the rate of decrease of the fluid pressure in the wheel brake cylinder tends to be lower than required, or the wheel brake cylinder pressure may temporarily change. Thus, the braking system suffers from inadequate control of the rate of decrease of the wheel brake cylinder pressure.
Further, the conventional braking system of the recirculating type or pressure-increase-by-pump type which uses a motor as the rotary pump actuator and has a diagnostic device for the motor suffers from a considerably large operating noise generated by the motor and the pump during an operation of the diagnostic device in which the motor and pump are operating at excessively high speeds, with the motor kept energized for a certain length of time.
In sum, all of the conventional braking systems described above suffer from a common problem that the operation of the pump is not sufficiently optimized during operation of the pressure control device or diagnostic device.